"GNOME theme engine designer Andrea Cimitan has implemented support for transparent widgets in the Murrine GTK theme engine, bringing Vista-like translucent glass effects to the GNOME desktop. Cimitan used RGBA colormaps to implement the feature and says that, with only 10 or 20 extra lines of code, translucency can easily be added to other theme engines that support RGBA. Cimitan says that the addition of translucency effects proves that critics of GTK are wrong. "[I]n the last week I've seen a lot of people claiming about 'lacks' of Gtk+ capabilities," wrote Cimitan in a blog entry. "Some of them still think that Gtk+ doesn't have RGBA support... Or it will require nasty hacks. This is absolutely false."

Well it's different from simply controlling the transparency of a whole window: now transparency can be on various elements of a GTK app. See for example the terminals in the second screenshot: the background is translucent, but the text isn't. I certainly can't do that on my compiz here.

Well, gnome-terminal atleast does support real translucent background if you are running a compositing manager such as Compiz-fusion. But you're right, this is not about window translucency, it's about the translucency being applied to single widgets such as buttons, images, scrollbars or whatever the author of the theme wants.